Laser Cooling 1. Doppler Cooling – optical molasses. 2. Magneto-optical trap. 3. Doppler temperature.

Doppler Cooling: How can a laser cool? Lab frame v 

v  Atom’s frame Doppler Cooling: How can a laser cool?

Lab frame v  Atom’s frame Lab frame, after absorption v-v recoil Doppler Cooling: How can a laser cool?

Lab frame v  Atom’s frame Lab frame, after absorption v-v recoil Doppler Cooling: How can a laser cool?

Lab frame v  Atom’s frame  Absorb a photon  atom gets momentum kick.  Repeat process at 10 7 kicks/s  large deceleration.  Emitted photons are radiated symmetrically  do not affect motion on average  Absorb a photon  atom gets momentum kick.  Repeat process at 10 7 kicks/s  large deceleration.  Emitted photons are radiated symmetrically  do not affect motion on average Lab frame, after absorption v-v recoil Doppler Cooling: How can a laser cool?

Lab frame v  Atom’s frame  Absorb a photon  atom gets momentum kick.  Repeat process at 10 7 kicks/s  large deceleration.  Emitted photons are radiated symmetrically  do not affect motion on average  Absorb a photon  atom gets momentum kick.  Repeat process at 10 7 kicks/s  large deceleration.  Emitted photons are radiated symmetrically  do not affect motion on average Lab frame, after absorption v-v recoil m/s m/s 2 87 Rb:  = -  I = I sat V doppler ~ 10 cm/s V recoil = 6 mm/s Doppler Cooling: How can a laser cool?

Magneto-Optical Trap (MOT) Problem: Doppler cooling reduces momentum spread of atoms only.  Similar to a damping or friction force (optical molasses).  Does not reduce spatial spread.  Does not confine the atoms.

Magneto-Optical Trap (MOT) Solution: Spatially tune the laser-atom detuning with the Zeeman shift from a spatially varying magnetic field. z B,  ~10 G/cm ~14 MHz/cm Problem: Doppler cooling reduces momentum spread of atoms only.  Similar to a damping or friction force (optical molasses).  Does not reduce spatial spread.  Does not confine the atoms.

Magneto-Optical Trap E ee gg 2-level atom 

E ee gg magnetic gradient +  z B Magneto-Optical Trap

 E ee gg 4-level atom |g   “F=0”, |e   “F=1” 4-level atom |g   “F=0”, |e   “F=1” magnetic gradient + z m F =0 m F =-1 m F =+1 B Magneto-Optical Trap

 E ee gg 4-level atom |g   “F=0”, |e   “F=1” 4-level atom |g   “F=0”, |e   “F=1” magnetic gradient + m F =0 m F =-1 m F =+1 z B Magneto-Optical Trap

 E ee gg 4-level atom |g   “F=0”, |e   “F=1” 4-level atom |g   “F=0”, |e   “F=1” magnetic gradient + m F =0 m F =-1 m F =+1   z B Magneto-Optical Trap

 E ee gg 4-level atom |g   “F=0”, |e   “F=1” 4-level atom |g   “F=0”, |e   “F=1” magnetic gradient + m F =0 m F =-1 m F =+1   z B -- ++ Magneto-Optical Trap

 E ee gg 4-level atom |g   “F=0”, |e   “F=1” 4-level atom |g   “F=0”, |e   “F=1” magnetic gradient + m F =0 m F =-1 m F =+1   z B -- ++ Magneto-Optical Trap

Magneto-Optical Trap (MOT)

~ 100  K

Magneto-Optical Trap (MOT)

Rb atoms

Francium MOT PROBLEM: Accelerator produces only 10 6 Fr atoms/s.  Very difficult to work with. SOLUTION: SOLUTION: Attach a Francium Magneto-Optical Trap to the accelerator.  Cold Francium is concentrated in ~1 mm 3 volume.  With T < 100  K, Doppler broadening is negligible.  Long integration times.  Minimally perturbative environment (substrate free).

Francium MOT PROBLEM: Accelerator produces only 10 6 Fr atoms/s.  Very difficult to work with. SOLUTION: SOLUTION: Attach a Francium Magneto-Optical Trap to the accelerator.  Cold Francium is concentrated in ~1 mm 3 volume.  With T < 100  K, Doppler broadening is negligible.  Long integration times.  Minimally perturbative environment (substrate free). MOT with ~ Fr atoms MOT collection efficiency ~ 1 %